Page 248 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

succession of electrofacies into log-based electrose- logical succession of facies and depositional environ-
quences, which can be interpreted in terms of possible ments. The breaks recognised from the logs should be
depositional environments, facies, facies successions added to the environmental interpretations and the princi-
and stratigraphic breaks, faults and unconformities. The pal electrosequences marked. The environmental! logic
evidence for the construction comes from each preceding should only continue benween the major breaks. This
step. For example, the lithology analysis indicates a document will now show the distinctive log-based
moderately thick sand interval with thick shale intervals electrosequences, stratigraphic breaks (disconformities,
above and below. The electrofacies annotation indicates unconformities} and structural breaks (faults) (Figure
log trends which show the sand to have a gradational 14,20). It is impossible to give details of all the reasoning
base and be part of a typical cleaning-up succession. The that goes into constructing a document of this sort. Many
top of the sand is separated from the overlying shale geological disciplines are involved. This section simply
by an abrupt break indicative of a flooding surface. The describes the system, here called electrosequence analy-
electrosequence therefore consists of a set of log trends sis, and how the body of data is built up to form the
indicating diminishing shale upwards into a clean sand interpretation tool. To use the tool one needs not just the
which is capped by an abrupt break indicating flooding. data from this book, but training and experience in all
Biostratigraphic analysis of sidewall cores in the shale fields of oil geology.
section shows the presence of marine fauna. The dipme- Previously, the geological document produced from
ter indicates good lamination throughout and no large this anlaysis would have been the final one and used for
sedimentary structures. The evidence for a shallow understanding basin development, correlation and
marine, prograding succession of facies and subsequent matching with the seismic. Today, it is not sufficient. The
flooding is building up. Does this fit with the interpreta- data must now be subjected to a sequence stratigraphic
tion of the preceding and following electrosequences? analysis. And this is the principal subject of the next
The final interpretation should be a sedimentologically chapter.

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